With airplanes, trains, buses, and other forms of passenger transportation, there is an obligation to transport the passengers safely. Especially with airplanes, since accidents thereof are enormous in damage, a high degree of caution must be paid for safety. Passengers using airplanes are thus subject to baggage inspection using an X-ray image taking device, body inspection using a metal detector or by body check, and, if necessary, questioning etc., in order to distinguish passengers with malicious intent and deny the use of an airplane to such passengers. However, in view of the large number of passengers and convenience to passengers, it is difficult to carry out strict inspections and questioning, which take a large amount of time, on all passengers. Meanwhile a passenger with malicious intent (such as a terrorist) will try to get past such inspections and bring a hazardous object inside a plane. Though problems will not occur in particular in regard to hazardous objects that can be discovered by currently used means of baggage inspection, etc., hazardous objects that cannot be detected by a metal detector or X-ray image taking, such as gasoline and other flammable liquids, etc., are comparatively difficult to detect. Gasoline and other hazardous liquids can be procured readily in the market, and when such a hazardous liquid is filled in a container (for example a PET bottle) for a commercially sold drink, it becomes difficult to distinguish between a true drink, and such an act can said to be a dangerous act that can readily be employed by a person with malicious intent. Countermeasures against such dangerous acts must thus be considered adequately.
For distinguishing between a hazardous liquid, such as gasoline, etc., and a drink having water as a main component, there are various distinguishing methods, such as sensory inspection methods, wherein the odor is smelled, etc. However, since rapidity of inspection is required of baggage inspection in using a plane, it is preferable for an inspection to be carried out rapidly in a non-contacting manner. As rapid, non-contacting inspection methods, there are methods that make use of differences in the dielectric constants of liquids. Whereas water is high in dielectric constant, gasoline and other hazardous liquids are generally low in dielectric constant. Such dielectric constant differences may thus be considered for use in judging the type of liquid.
In Patent Document 1 is disclosed a method and device for judging the type of liquid from the exterior of a container. With the art described in this document, the interior of a container is filled with a liquid, a pair of electrodes are positioned outside the container so as to sandwich at least a part of the liquid, and the capacitance of a capacitor formed by these electrodes is measured to judge the type of the liquid. In using such an art, the capacitance in the case where the liquid is water and the capacitance in the case where the liquid is a hazardous liquid (for example, gasoline) are measured in advance, and by then positioning a container filled with a liquid of unknown content between the electrodes and measuring the capacitance in this state, the type of liquid inside the container can be judged instantly and accurately.
Patent Document 1 Japanese Published Unexamined Patent Application No. 2001-272368